CN101657891A - Convex die attachment method - Google Patents

Abstract

A method for assembling a microelectronic device is provided comprising the step of adhering a die to a substrate using a convex die attachment process. The convex die attachment process generally comprises a) providing a die having an underfill material thereon, b) picking up and inverting the die, c) heating the underfill until it liquefies at least slightly and forms a convex surface, and d) placing the die on a substrate.

Description

Convex Die Attach Method

Cross References to Related Applications

This application claims benefit under 35 U.S.C § 119(e) of U.S. Provisional Patent Application No. 60/894,574, filed March 13, 2007, entitled "Convex Die Attachment Method" priority, the disclosure of which is incorporated herein by reference.

field of invention

The present invention relates to electronic packaging. More particularly, the present invention relates to a method for assembling an electronic package that employs the application of a no-flow underfill material to the die prior to placing the die on a substrate.

Background of the invention

In conventional manufacturing processes, individual dies containing electrical components are formed in large numbers on a single large "wafer," typically silicon, as the semiconductor material. Individual dies have small metal pads that serve as electrical connections. The individual dies are then cut out of the wafer and connected to the housing using small wires leading from the pads. These wires connect to pins on the outside of the housing, which are then connected to a substrate, such as a printed circuit board (PCB).

A newer process for chip fabrication called "flip-chip" is advantageous because it does not require any wire bonding and is often used in semiconductor devices such as IC chips. A flip chip is a die that is simply flipped over so that the side of the die containing the circuitry is closest to the mounting substrate. In the final step of processing the wafer, solder balls or "bumps" are placed on the die pads, which are used to connect directly to the corresponding bonding connectors on the substrate circuit.

Flip chip processing is similar to conventional IC manufacturing with some slight modifications. Immediately after the wafer is fabricated, a small solder dot is placed on each pad. Individual dies are then cut (diced) from the wafer. The flip chip is connected to the substrate by flipping the chip so that the solder balls are placed down to the underlying electronics or connectors on the circuit board. The solder is then re-melted (reflowed) to create an electrical connection between the circuitry on the die and the substrate.

The reflowed solder bumps create mechanical and electrical connections between the contact areas of the die and the contact areas of the substrate. However, due to the nature of the solder, this mechanical connection is relatively weak and prone to deformation or cracking during the die's natural thermal cycling. In addition, the underside of the die surface remains exposed, suspended away from the surface of the mounting substrate by the reflowed solder bumps. An electrically insulating adhesive, often called an underfill, is typically injected into this space and cured to provide a stronger mechanical connection, provide thermal bridging, and ensure that the solder joint does not can be stressed due to the difference in coefficient of thermal expansion between the die and the substrate.

The underfill material flows by capillary action between the die and the substrate, and thus requires considerable time and needs to be applied from multiple points to ensure that there are no unfilled voids between the die and the substrate. The underfill material is generally epoxy based and has the right viscosity to flow properly and is mechanically strong after consolidation/curing. After the shot is applied, the underfill is heated to drive out any solvents and/or to cure the underfill composite. This can be done prior to, or as part of, the solder reflow process.

The underfill material often does not completely fill the space between components, leaving air under the die. Catastrophic failure of the part can result when the heat from the reflow oven causes the trapped air to expand and burst the underfill or die.

Applying the underfill material manually with a syringe is a time-consuming and labor-intensive step compared to other automated processes. One attempt to eliminate this step involves coating the substrate with an underfill material in place before placing the die on the substrate. This is commonly referred to as no-flow primer fill and is shown in Figures 1A-1C. After the die is fabricated, a small solder dot is placed on each pad. Individual dies are then cut (diced) from the wafer. Die 10 containing solder bumps 20 is then inverted into a position ready to be aligned and placed on a substrate, as shown in FIG. 1A . FIG. 1B shows die 10 in an inverted position, ready to be aligned and placed on substrate 40 , which has been coated with underfill compound 30 . Die 10 is aligned so that solder bumps 30 are aligned with connectors 50 on substrate 40 . In FIG. 1C , flip chip 10 is placed and connected to substrate 40 such that solder balls 20 are placed down onto connectors 50 of an underlying electronic device or circuit board 60 and the solder is reflowed. Although this process automates the application of the underfill material, problems remain with uneven wetting of the underfill material on the substrate and solder bumps preventing air from There is trapped air 60 between the sheets.

Summary of the invention

In a first aspect of the invention, there is provided a method for assembling a microelectronic device, the method comprising the step of attaching a die to a substrate using a convex die attach process.

In a second aspect of the invention there is provided a method for forming an electronic assembly, the method comprising:

a) providing a die with an underfill material thereon;

b) pick up and flip the die;

c) heating the underfill material until it at least slightly liquefies and forms a convex surface, and

d) Placing the die on the substrate.

In one embodiment of the invention, the die includes microelectronic elements. In another embodiment of the invention, the die includes external electrical connections. In yet another embodiment of the invention, the die includes solder bumps, and in another embodiment of the invention, the underfill material substantially surrounds the solder bumps. In a further embodiment of the invention, the substrate includes pads for connection to solder bumps.

In another embodiment of the present invention, step a) of the method comprises:

a1) Provide wafers;

a2) forming solder bumps on said wafer;

a3) coating the wafer with bumps with an underfill material comprising epoxy resin and a solvent;

a4) drying the underfill material to remove substantially all of the solvent;

a5) Dicing the wafer into individual dies.

In other embodiments of the present invention, the step of drying the underfill material to remove substantially all of the solvent is performed under vacuum and/or the underfill material is heated to dry the underfill material.

In one embodiment of the invention, the step of picking up the die and flipping the die includes picking up the die using a heated die bonder that provides heating through the body of the die step heat. In a variant embodiment of the invention, the underfill material is heated via a flow of hot air directed towards the die.

In another embodiment of the invention, the step of positioning and placing the coated die on the substrate comprises:

d1) Aligning the solder bumps on the die with the corresponding pads on the substrate;

d2) placing the die on the substrate;

d3) allowing the underfill material to wet the substrate and substantially fill the space between the die and the substrate;

d4) Heating the assembly to solidify the underfill material.

In one embodiment of the invention, the underfill material is cured by heating the assembly. In another embodiment of the present invention, the step of heating the assembly to cure the underfill material includes heating the substrate to a temperature sufficient to reflow the solder.

In another embodiment of the present invention, the underfill composite comprises epoxy resin, solvent, curing agent, and optionally flux. In yet another embodiment of the present invention, the curing agent comprises a heat latent curing agent. In another embodiment of the present invention, the epoxy resin comprises a solid bisphenol A or bisphenol F epoxy resin. In yet another embodiment of the present invention, the melting point of the epoxy resin is less than about 100°C. In a further embodiment of the invention, the solvent comprises dichloromethane.

In another embodiment of the invention, the substrate contains another die to form a stacked chip assembly. In further embodiments of the invention, the stacked chip assembly includes a plurality of dies. In another embodiment of the invention, the substrate is coated with an underfill composition prior to placement of the underfill coated die.

Brief description of the drawings

Figure 1 shows a prior art microelectronic assembly.

Figure 2 shows a convex die attach process in one embodiment of the invention comprising (a) a coated flipped die, (b) a heated die with a convex formed thereon , and (c) placing the die on the substrate.

Figure 3 shows the complete fabrication and soldering process according to one embodiment of the invention comprising (a) a wafer with solder bumps, (b) a wafer with an underfill material applied, (c) a wafer with Wafer with dried underfill material, (d) wafer being sliced, (e) die picked up and flipped, (f) die placed on substrate, and (g) die adhered to solder Substrate with reflowed and cured underfill material.

Detailed Description of the Invention

There are various methods used in the manufacture of microelectronic components for producing silicon wafers, attaching solder bumps, and dicing the wafers into individual dies. The methods of the various embodiments of the invention can be readily integrated into existing processes as will be recognized by those skilled in the art.

In a first aspect of the invention, the die is coated with an underfill material. The underfill material preferably contains epoxy resin. The underfill material may optionally contain one or more of curing agents, solvents, flux solutions, and fillers. In one embodiment of the invention, a wafer containing a plurality of dies is coated and then diced into individual coated dies. In another embodiment of the invention, the dies are coated individually after being diced from the wafer.

In another embodiment of the present invention, an important characteristic of the underfill composite is its liquefaction temperature. The liquefaction temperature is the temperature at which the solid underfill material liquefies and begins to flow, forming a convex surface on it due to gravity when the die is flipped over. In one embodiment of the invention, this temperature will fall within the common operating and processing range for flip chip applications and can vary from about 20°C to about 270°C. In a preferred embodiment of the invention, the liquefaction temperature will be in the range of from about 40°C to about 150°C, and most preferably from about 80°C to about 120°C.

The underfill composite employed in the method of the present invention may comprise any underfill composite suitable for the heating/liquefaction step as described herein. The underfill composite is tuned to have the proper viscosity, liquefaction temperature, and any other properties that may be required for a particular application. In a preferred embodiment of the present invention, the underfill composite comprises an epoxy-based underfill comprising a thermal latent curing agent. The thermal latent curing agent allows the underfill material to be heated on the die without causing curing, allowing the underfill material to remain uncured throughout the pick/heat/place steps. After the die is placed on the substrate, the underfill material is then heated above the initial temperature of the curing agent to initiate curing of the epoxy. Curing begins when the epoxy polymerizes or crosslinks to such an extent that the tack increases considerably.

In the most preferred embodiment of the invention, the underfill composite comprises a bisphenol A or bisphenol F solid epoxy resin with a melting point temperature below about 100°C, and a thermal latent curing agent, such as Stapleton's U.S. Patent Application Publication No. The curing agent described in No. 2008/0012124 having a curing initiation temperature above about 150°C.

In most flip-chip devices, the size of the solder balls ranges from 25 μm to 500 μm. Depending on the application, the underfill material can completely cover the solder ball or only a portion thereof. Thus, in one embodiment of the invention, the thickness of the underfill material may vary from 0.1 μm to 10 mm, preferably between 25 μm to 1 mm, and ideally between 100 μm to 400 μm. However, it should be recognized that the geometry of the microelectronic component assembly and the nature of the underfill material will dictate the final thickness applied.

In embodiments of the present invention where the underfill material contains a solvent, prior to the placing step, a drying step is required to remove the solvent from the underfill material. The drying step includes, for example, heating the underfill material to evaporate the solvent, or placing the underfill material under vacuum to remove the solvent from the underfill material.

Once the die is coated with the underfill material and any solvent is removed, the coated die can optionally be stored for a period of time until the microelectronic assembly is to be constructed.

In a further embodiment of the invention, as shown in FIGS. 2A-2C , die 10 coated with underfill material 30 is picked up and turned over so that solder balls 20 and underfill composite composition 30 are down. Die 10 coated with underfill material 20 is then heated. After heating to a sufficient temperature, ie, at or near the liquefaction temperature of the underfill material, gravity and surface tension will cause the underfill material to form a convex surface, as shown in Figure 2B. In embodiments of the invention, the shape of the convexity can be tuned using temperature, coating thickness, viscosity, and surface energy of the underfill material. The heated die 10 including the underfill compound 30 having a convex surface is then positioned and placed on the substrate 40 . The convex shape of underfill material 30 allows air 60 to escape, preventing air from being trapped between die 10 and substrate 40 .

In one embodiment of the invention, dies are facilitated to be picked, heated and placed by using a heated die bonder. A die bonder typically uses a placement head that picks up the die via suction, aligns the board and die placement, then places the die on the board and stops the suction to release the die. Heated die bonders, such as those sold by Datacon (Datacon North America, Trevose, PA 19053), allow the die and/or substrate to be heated simultaneously as the die is placed. Means for heating the die include conductively heating the die by heating the placement head, heating the substrate via conduction heat from the die bonder, or convective heating of the plate and/or substrate via a stream of hot air.

The die is placed such that the solder balls contact corresponding connector pads on the substrate to allow electrical interconnection between the die and the substrate, preferably a printed circuit board. The convex shape of the underfill material leaves room for air to escape when the underfill material wets the substrate. This prevents the undesirable entrapment of air discussed herein. The underfill material wets the surface of the substrate and substantially fills the area between the die and the substrate, and around the solder balls. In a preferred embodiment of the invention, the substrate completely surrounds the solder balls without leaving any voids in the underfill material.

In an alternative embodiment of the present invention, the step of picking and placing is accomplished using an inclined or uneven pick and place head. This will allow the apex of the raised coating to be placed off-center relative to the die when the die is placed. Similarly, in some cases it may be advantageous to keep the substrate at an angle relative to the die to provide the same off-center alignment.

In another embodiment of the invention, both the die and the substrate are heated during the placing step. Heating the die and board ensures that the underfill material remains liquid until it has had a chance to fully wet the surface of the substrate. Typically, the temperature will be within the normal operating temperature range of about 20°C to about 270°C. In a preferred embodiment of the invention, the temperature is between about 40°C and about 150°C, and desirably between about 80°C and about 120°C.

After the die is placed on the substrate, the microelectronic assembly is heated to reflow the solder and cure the underfill compound. In an alternative embodiment of the invention, the underfill material is cooled after placing but before reflowing the solder. This allows the underfill compound to recure and hold the component together. This can be advantageous when there is a time delay between placement and reflow, or when components must be moved or stored between these steps. In an additional embodiment of the invention, the assembly is placed in a post-bake process to allow the underfill material to cure at a temperature below the reflow temperature of the solder.

In another embodiment of the invention, both the substrate and the die are coated with an underfill material. The coating of underfill material on the substrate provides a uniform contact surface by covering any surface features of the substrate such as solder mask or protruding electrical interconnects. The composition of the underfill material applied on the substrate may be different from that used to coat the die. However, in a preferred embodiment of the invention, the underfill compound on the substrate is substantially the same as the underfill compound on the die. In addition, coating the substrate provides underfill material to underfill material contact when the die is placed, which improves wetting and further reduces the possibility of entrapped air during placement.

In another embodiment of the invention, the substrate contains another die. In this embodiment, a microelectronic assembly is formed by stacking dies one on top of the other with electrical interconnections to each other. It is within the scope of this embodiment of the invention to provide a plurality of stacked dies in an assembly prepared according to the methods of the various embodiments herein.

example

In a first exemplary embodiment of the present invention, as shown in FIGS. 3A-3G , the method of the embodiment of the present invention is incorporated into a microelectronic component manufacturing process. In FIG. 3A, the process begins with a wafer 100 having a plurality of solder bumps 120 formed thereon. In FIG. 3B the wafer 100 is coated with an underfill material 130 that completely covers the solder bumps 120 . The underfill material 130 includes solid epoxy resin, thermal latent curing agent, solvent and flux solution according to Table 1 .

Table 1 - Underfill Composite Materials

^* LORD Curative contains the curing agent described in US Patent Application No. 2008/0012124 to Stapleton.

As shown in FIG. 3C , the underfill composition 130 is then hardened by removing the coating solvent using heat and a slight vacuum to produce a non-tacky dry coating on the die 100 that completely covers the Solder bumps 120 . The wafer 100 is then diced into individual dies 110, as shown in FIG. 3D.

As shown in FIG. 3E , individual dies 110 are then picked up by heated die bonder 170 , at which time solid uncured resin 130 liquefies to create a convex surface. In FIG. 3F, the die 110 with the low viscosity convex surface is then aligned and placed onto a heated plate 140, allowing the resin 130 to wet the surface of the plate 140 during the bonding process. The board 140 is simultaneously heated to a temperature during placement which is below a temperature at which the board 140 may be damaged or the resin may begin to cure.

Die 110 and board 140 then pass through a reflow oven to form physical connections between 120 and electrical components 150 on board 140 . The assembly is then post baked at 180-200°C for 1 hour to ensure complete curing of the underfill compound.

While the invention has been described with reference to specific embodiments, it should be recognized that these embodiments are merely illustrative of the principles of the invention. Those skilled in the art will appreciate that the apparatus and method of the present invention may be constructed and implemented in other forms and implementations. Therefore, the description herein should not be construed as limiting the invention, as other embodiments also fall within the scope of the invention.

While the invention has been described with reference to specific embodiments, it should be recognized that these embodiments are merely illustrative of the principles of the invention. Those skilled in the art will appreciate that the composite materials, devices and methods of the present invention may be constructed and implemented in other forms and embodiments. Accordingly, the description herein should not be read as limiting the invention, since other embodiments also fall within the scope of the invention as defined by the appended claims.

Claims (22)

1. method that is used to form electronic building brick, it comprises:

A) provide the tube core that has underfill thereon;

B) the described tube core of picking up and overturn;

C) the described underfill of heating liquefies at least slightly and forms a convex surface up to described underfill, and

D) described tube core is placed on the substrate.

2. the method for claim 1, wherein said tube core comprises microelectronic element.

3. the method for claim 1, wherein said tube core comprise outside electrical connection.

4. the method for claim 1, wherein said tube core comprises solder projection.

5. method as claimed in claim 4, wherein in step a), described underfill is haply around described solder projection.

6. method as claimed in claim 4, wherein said substrate comprise and are used for the pad that is connected with described solder projection.

7. the method for claim 1, wherein step a) comprises:

A1) provide wafer;

A2) on described wafer, form solder projection;

A3) use the underfill that comprises resin and solvent to apply the described bump wafer that has;

A4) dry described underfill is to remove whole solvents haply;

A5) described wafer is cut into other tube core.

8. method as claimed in claim 7, wherein step a4) under vacuum, carry out.

9. method as claimed in claim 7 is wherein at step a4) in, described underfill is heated with the described underfill of drying.

10. the method for claim 1, wherein step b) comprises: use the tube core connector of heating to pick up described tube core, described connector provides heat by the main body of described tube core for step c).

11. the method for claim 1, wherein in step c), described underfill is heated via the thermal air current of the described tube core of guiding.

12. the method for claim 1, wherein step d) comprises:

D1) the described solder projection on the described tube core is aimed at corresponding bonding pad on the described substrate;

D2) described tube core is placed on the described substrate;

D3) the wetting described substrate of the described underfill of permission is also filled the space between described tube core and the described substrate haply;

D4) heat described assembly, with fixed described underfill.

13. technology as claimed in claim 12 is wherein in steps d 4) in, described underfill is cured.

14. technology as claimed in claim 12, wherein steps d 4) comprising: described substrate is heated to the temperature that is enough to make described solder reflow.

15. technology as claimed in claim 1, wherein said bottom filled composite materials comprises epoxy resin, solvent, curing agent, and selectively comprises solder flux.

16. technology as claimed in claim 15, wherein said curing agent comprise the heat curing agent of hiding.

17. technology as claimed in claim 16, wherein said epoxy resin comprises solid-state bisphenol-A or bisphenol F epoxy resin.

18. technology as claimed in claim 16, the fusing point of wherein said epoxy resin is less than about 100 ℃.

19. technology as claimed in claim 15, wherein said solvent comprises carrene.

20. technology as claimed in claim 1, wherein said substrate comprises the chip assembly that another tube core piles up with formation.

21. technology as claimed in claim 20, the wherein said chip assembly that piles up comprises a plurality of tube cores.

22. technology as claimed in claim 1 wherein, scribbles in placement before the tube core of underfill, described substrate is scribbled underfill composition.

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